This invention relates to tactile sensor arrays and, more particularly, to robotic systems which incorporate such arrays in a moving robot part and to manufacturing methods utilizing such robots.
It is becoming evident that tactile sensing would greatly improve the manipulative capacity of robots. See, for example, an article by L. D. Harmon, International Journal of Robotics, Vol. 1, p. 3 (1982) and a book by R. P. Paul, Robot Manipulators, Chapter 9, MIT Press (1981). In contrast to single-point contact sensing, robotic tactile sensing usually means sensing patterns of touching; i.e., continuous sensing of forces in an array.
Many types of tactile array sensors have been proposed. See, for example, an article by D. Hillis, International Journal of Robotics, Vol. 1, p. 33 (1982) and the copending application of J. F. Jarvis et al, application Ser. No. 434,876 filed on Oct. 18, 1982, i.e. U.S. Pat. No. 4,539,554, and assigned to the assignee hereof. Anisotropically resistive materials, semiconductor piezoresistors, piezoelectric transducers, capacitive and photoelectric sensors all show promise for special applications. Disadvantages exhibited by one or another of these devices include hysteresis, fragility, low dynamic range, susceptibility to external influences, etc. A common limitation of all these devices is the lack of torque sensing and, in most cases, also tangential force sensing.
Although torque and tangential force sensing are very useful properties for robotic applications, the known tactile sensors do not provide torque sensing primarily because of the nature of the transduction effect. In fact, the transduction is generally at the atomic level, where the transduction effect (e.g., piezoresistive, magnetostrictive, piezoelectric, etc.) couples one form of energy to another. However, apart from certain effects seen in single crystals (which are not sufficiently robust for robotics) a microscopic coupling between two physical quantities has generally small anisotropic coefficients. Thus, it is very unlikely that any physical effect would have several large components of the transduction tensor, which limits the applicability of microscopic effects to the detection of one or possibly two components of the applied force.
An additional inherent limitation of microscopic transduction effects is the trade-off between sensitivity and fragility. For robotic applications, fragility is a serious consideration. For example, strain gauges, which are highly sensitive transducers, are difficult to implement on robot fingertips. Thus, it is perhaps not surprising that the only known commercial tactile sensor for robotics is based on macroscopic transduction. In this device, (which is labeled the Touch Sensor.TM. by its manufacturer, the Lord Corporation of Cary, N.C.) the sensing element is composed of three parts: a light emitting diode (LED), a photodetector and a flexible surface. When the surface is depressed by a force being sensed, the optical path between the LED and the photodetector is interrupted so that the current decrease in the photodetector is a measure of the force exerted on the surface.